[linux-libre] add g_dbgp
Many ARM device have a USB OTG port, so the port can act as an usb device.
The g_dbgp permits that port to get "USB debug" logs.
Linux(-libre), grub, and coreboot/libreboot can output their logs to the "USB debug" port,
that is really useful when the machine has no serial port, like most x86 recent laptops.
CONFIG_USB_G_DBGP should be set as module (=m).
Once selected, you have the choice between CONFIG_USB_G_DBGP_SERIAL and USB_G_DBGP_PRINTK.
The later prints what the target machine(like the x86 laptop logs) on dmesg, so CONFIG_USB_G_DBGP_SERIAL is preferred since it will create a serial port where the logs are available and won't mess with the "dmesg buffer".